The present invention relates to an image display apparatus such as a liquid crystal projector and a discharge lamp lighting device used in the image display device, and more specifically to a technique for superposing pulse waves of a lamp current.
A high voltage discharge lamp (discharge lamp) having high conversion efficiency and functioning as a point source of light such as a metal halide lamp or a high voltage mercury lamp is now used as a light source for an image display device such as a liquid crystal projector or the like. To light a high voltage discharge lamp, a voltage and a current necessary for lighting the lamp are supplied from a dedicated discharge lamp lighting device. Sometimes the flickering phenomenon may occur in a discharge lamp due to movement of a starting point of a discharge arc while the discharge lamp is lighted. To stabilize the operation of a discharge lamp lighting device, several techniques have been proposed.
Japanese Patent Laid-open No. 5-74583 discloses a technique in order to provide a HID (high intensity discharge) lamp lighting device with high lighting efficiency by maintaining the power consumption of the HID constant. In this technique, the power consumption of the HID lamp is computed based on a lamp current flowing in the HID lamp and a voltage difference between both ends of the HID lamp. In addition, a value of current supplied to the HID lamp is controlled according to a difference between the computing result and a preset value. The computing is performed by a microprocessor.
International Publication No. WO 95/35645 discloses a configuration in which an AC lamp current on which a current pulse with a predetermined cycle is superposed is supplied to a high voltage lamp in order to suppress its flickering occurring when the lamp is lit.
Japanese Patent Laid-open No. 2004-281381 proposes a lamp current control circuit for stabilizing power at a constant level by suppressing the flickering of a high voltage discharge lamp during lighting and also keeping brightness of the lamp at a constant level for the purpose of stabilizing control and extending the life of the lamp. “Detailed Descriptions of the Embodiments” in the publication teach that “the amplitude waveform of a lamp flicker reduction step signal 57 superposed on an AC lamp current provides a step signal 18 bout well balanced in the vertical direction even when adjusted with a resister 31a, and an average value of the waveforms remains unchanged. As a result, the lamp current remains unchanged, and a lamp can be lit more smoothly while reducing flickering in the lamp” (paragraph [0051]). In addition, a waveform of the step signal is shown, for instance, in FIG. 3 in the document.